1. Field of the Invention
The present invention relates to a high efficiency prism coupling device used for optical integrated circuits, optical sensors, and other optical devices. The present invention also relates to a method for producing the prism coupling device.
2. Description of the Related Art
A prism coupling device comprises a prism and a light guide layer to which an optical beam is guided through the prism so that the prism is optically coupled with the light guide layer. The prism is directly press bonded to the surface of the light guide layer to easily obtain a high coupling efficiency between the prism and the light guide layer.
However, in accordance with the optical coupling structure in which the prism is directly press bonded to the optical waveguide layer, the device is easily affected by mechanical vibrations or impacts, which causes the instability of the coupling efficiency of the device.
To cope with the problem mentioned above, the prism is disposed apart from the optical waveguide layer surface forming a predetermined gap therebetween instead of being directly bonded to the waveguide layer surface. The structure is more precisely described below.
An example of the structure comprises a substrate on which an optical waveguide layer having a high refractive index is formed. On the surface of the high refractive index waveguide layer, a gap adjuster layer having a low refractive index is formed. Further, on the low refractive index gap adjuster layer, a prism having a high refractive index is secured through a cement layer having a high refractive index used as an adhesive agent to bond the prism to the gap adjuster layer.
Due to the arrangement of the low refractive index gap adjuster layer between the prism and the waveguide layer, the prism is not easily affected by the vibrations or impacts, which makes it possible to maintain the gap between the prism and the waveguide layer constant so that the optical coupling efficiency becomes stable.
However, the incident beam to be coupled with the waveguide layer is functioned by the optical element (the prism in this example) disposed on the waveguide layer so that the beam is converged, reflected, passed through or coupled. therefore, to heighten the coupling efficiency, it is necessary to introduce the beam to the waveguide layer from a predetermined direction at an angle .theta. with respect to the normal line of the waveguide layer surface. For this purpose, it is necessary to guide the incident beam to an edge of the cement layer of the structure mentioned above.
The reason why the beam is to be guided and impinged to the edge of the cement layer is as follows.
It the beam is guided to the center or around the center of the cement layer, the beam is transmitted to the waveguide layer through the cement layer first and after that the beam is recoupled with the prism and emitted out of the device, which is called decoupling phenomenon and which lowers the optical coupling efficiency. On the other hand, if the beam is guided to the edge of the cement layer, the beam is not recoupled with the prism so that it becomes possible to avoid the decoupling phenomenon.
However, the position of the edge of the cement layer which is used as an adhesive bonding means is not always constant so that it is difficult to predetermine the position of the edge. Also, it becomes necessary to change the incident point of the beam according to the position of the edge of the cement layer. However, it is very troublesome to change the position of the incident point of the beam since the optical system is constituted so that the beam is guided from a predetermined direction.